I. DISINFECTANTS 



The difficulty of defining disinfectants has just been 

 pointed out. To give some precision to the definition 

 let us assume that we use as a standard method the well- 

 known technique of the phenol coefficient. Accordingly, 

 a substance is considered a disinfectant if 5 cc. of the 

 most concentrated of its commonly used solutions, mixed 

 with l^ cc. of a 24-hour culture of the test organism, 

 reduces the number of viable cells so far that, after one 

 hour, a loojDful of the suspension transferred into broth 

 will not cause growth. 



This definition excludes such compounds as CuSOi, 

 ZnS04, NaoSOs, all the dyes, sulfanilamide, quinine and 

 practically all the therapeutic agents. These substances 

 are, therefore, antiseptics, and will be considered in the 

 second section of this review. The first section, which 

 will treat of disinfectants proper, will include alcohol, 

 phenol, formaldehyde, hydrogen peroxide, the silver and 

 mercury salts, the mineral acids and the alkalies, the 

 halogens, a few neutral salts and complex organic com- 

 pounds. The soaps will also be included in the first sec- 

 tion, though from purely practical considerations, be- 

 cause, while soaps really are antiseptics, they are never 

 used as such. 



I. SELECTIVITY OF DISINFECTANTS 



It has been realized from the earliest studies of disin- 

 fection that not all bacteria react alike with the same 

 disinfectant. Some groups, or some species, are more 

 sensitive or more tolerant than others. An outstand- 

 ing example of tolerance is that of mycobacteria to- 

 wards alkali and chlorine. Another practically impor- 

 tant example of selectivity is that of the Gram-negative 

 and Gram-positive species which often react differently 

 with a given disinfectant. For this reason it has be- 



